This invention relates generally to the manufacture of enhanced tubes and more particularly to a method for measuring the pore size in an externally enhanced evaporator tube.
In an evaporator of certain refrigeration systems a fluid to be cooled is passed through heat transfer tubing while refrigerant in contact with the exterior of the tubing changes state from a liquid to a vapor by absorbing heat from the fluid within the tubing. The external and internal configuration of the tubing is important in determining the overall heat transfer characteristics of the tubing. For example, it is known that one of the most effective ways of transferring heat from the fluid within the tube to the boiling refrigerant surrounding the tube is through the mechanism of nucleate boiling.
It is theorized that the provision of vapor entrapment sites or cavities cause nucleate boiling. According to this theory the trapped vapor forms the nucleus of a bubble, at or slightly above the saturation temperature, and the bubble increases in volume as heat is added until surface tension is overcome and a vapor bubble breaks free from the heat transfer surface. As the vapor bubble leaves the heat transfer surface, liquid refrigerant enters the vacated volume trapping the remaining vapor and another vapor bubble is formed. The continual bubble formation together with the convection effect of the bubbles traveling through and mixing the boundary layer of superheated liquid refrigerant, which covers the vapor entrapment sites, results in improved transfer. U.S. Pat. No. 3,301,314 discloses a heat exchange surface having a number of discrete artificial nucleation sites.
It is further known that a vapor entrapment site produces stable bubble columns when it is of the re-entrant type. In this context, a re-entrant vapor entrapment site is defined as a cavity or groove in which the size of the surface pore or gap is smaller than the subsurface cavity or subsurface groove. U.S. Pat. Nos. 3,696,861 and 3,768,290 disclose heat transfer tubes having such re-entrant type grooves.
Also, it is known that an excessive influx of liquid from the surroundings can flood or deactivate a vapor entrapment site. In this regard, a heat transfer surface having subsurface channels communicating with the surroundings through surface openings or pores having a specified "opening ratio" may provide good heat transfer and prevent flooding of the vapor entrapment site or subsurface channel.
In regard to the interior surface configuration of a heat transfer tube it is known that providing an internal rib on the tube may enhance the heat transfer characteristics of the tube due to the increased turbulence of the fluid flowing through the ribbed tube.
As disclosed in U.S. Pat. Nos. 4,425,696 and 4,438,807 assigned to the present assignee, and incorporated by reference herein, an internally and externally enhanced heat transfer tube, having an internal rib and an external groove--commonly referred to as a subsurface channel--communicating with the surrounding liquid through surface openings, i.e. pores, may be manufactured by a single pass process with a tube finning machine. According to the disclosed process a grooved mandrel is placed inside an unformed tube and a tool arbor having a tool gang thereon is rolled over the external surface of the tube. The unformed tube is pressed against the mandrel to form at least one internal rib on the internal surface of the tube. Simultaneously, at least one external fin convolution is formed on the external surface of the tube by the tool arbor with the tool gang. The external fin convolutions form subsurface channels therebetween. The external fin convolution has depressed sections above the internal rib where the tube is forced into the grooves of the mandrel to form the rib. A smooth roller-like disc on the tool arbor is rolled over the external surface of the tube after the external fin is formed. The smooth roller-like disc is designed to bend over the tip portion of the external fin to touch the adjacent fin convolution to form enclosed subsurface channels only at those sections of the external fin which are not located above the internal rib. The tip portion of the depressed sections of the external fin, which are located above an internal rib, are bent over but do not touch the adjacent convolutions thereby leaving pores which provide fluid communication between the fluid surrounding the tube and the subsurface channels.
The performance of the foregoing tube is critically dependent on the size of the subsurface channels and pores on the surface of the tube, and this performance may be enhanced by the internal ribs. It is therefore important to maintain a consistent subsurface channel size and pore size during the manufacturing process. Normal variations in subsurface channel size and surface pore size do occur, however, due to tool wear, dimensional and material variations in the tube lengths, and machine tolerances. In order to account for these variables and to maintain a consistent pore size, it is necessary to measure the pore size on each tube produced and adjust the finning machine to maintain the correct subsurface channel and pore sizes. However, the prior methods of having an operator randomly selecting manufactured tubes and optically checking the pore size of the selected tube under a microscope or using an image analyzer to compare the area of a pore in a photograph of a tube with the known area of a reference photograph were time consuming and did not provide the quality and quantity of tubes necessary for a manufacturing process. Not only were these very laborious and expensive practices, but they also cannot be used to check each and every tube in a manufacturing process.
Thus, there is a clear need for a method for measuring the size of the surface pores in an enhanced tube that will, to a large extent, overcome the inadequacies that have characterized the prior art.